To meet the recent demands for saving energy, power modules employable for power converters have been increasing the fields of application thereof. A conventional power converter module structure is shown in FIG. 17. The power converter module shown in FIG. 17 includes converter section 100, brake section 200, and inverter section 300. In inverter section 300, insulated gate bipolar transistor (hereinafter referred to as “IGBT”) 301 and a free wheeling diode (hereinafter referred to as “FWD”) 302 are connected in opposite parallel to each other. Usually, FWD 302 used in inverter section 300 conducts a reverse recovery mode of operation, through which FWD 302 recovers a reverse voltage blocking state from a forward current conduction state. Since FWD 302 is liable to cause breakdown during its reverse recovery mode of operation, it is required for FWD 302 to exhibit a certain reverse recovery withstand so that the device will hardly ever break down.
Now the process of breakdown caused during the reverse recovery mode of operation will be described below in connection with the behaviors of the internal carries and with reference to FIG. 18. FIG. 18 is a cross-sectional view of a semiconductor substrate in a conventional FWD. The conventional FWD has a general structure that employs n-type silicon semiconductor substrate (hereinafter referred to as “n-type substrate”) 1. In the first major surface portion of n-type substrate 1, p-type anode diffusion region 2 is formed. Anode electrode 3, made of an Al—Si alloy or like alloy, is made to be in ohmic contact with p-type anode diffusion region 2. On the back surface of n-type substrate 1, n+-type diffusion layer 4, the surface impurity concentration thereof is high enough to realize an ohmic contact, is formed. Cathode electrode 5 formed of a metal film laminate including a Ti film, a Ni film, and a Au film is formed on n+-type diffusion layer 4.
In the peripheral surface portion on the anode side of the FWD, edge termination region 6 is formed. Edge termination region 6 has an electric field relaxation structure such as guard ring structure 6-1, a field plate structure and a RESURF structure that facilitates securing the breakdown voltage of the FWD. Since a main current flows through p-type anode diffusion region 2 on the inner side of edge termination region 6, the region including p-type anode diffusion region 2 is called “active region 7”. In the following descriptions, the boundary between edge termination region 6 and active region 7 is set on outer edge 12 of p-type anode diffusion region 2.
The load of the power module that mounts the FWD's having the structure described above is the inductance that is typically a motor. As described in FIG. 17, a reflux current flows also through the FWD's in response to the ON and OFF of the IGBT's. Initially, the FWD is in the blocking state, in which the FWD is biased reversely. When a reflux current flows, the FWD having the structure described above is biased forward.
Referring again to FIG. 18, as the hole potential in p-type anode diffusion region 2 in the FWD biased forward exceeds the diffusion potential of the pn-junction (internal potential), holes are injected from p-type anode diffusion region 2 to n−-type layer (the same with the n-type substrate) 1 as minority carriers. As a result, conductivity modulation occurs corresponding to the concentration of hole carriers injected heavily to n−-type layer 1 and the concentration of electron carriers (majority carriers) increases. Therefore, the FWD exhibits a forward characteristic, in which the resistance reduces greatly and the forward current increases rapidly as the well-known forward I-V curve of a diode indicates.
Then, as the FWD is biased in reverse, the excess minority carriers (holes) and majority carriers (electrons) caused by the conductivity modulation and remaining in n−-type layer 1 are recombined with each other or ejected, making a depletion layer expand in n−-type layer 1. As the depletion layer expands to the utmost thereof, the FWD is in a voltage blocking state. This process is called the “reverse recovery”. The carrier ejection process described above and caused during the reverse recovery is called the “reverse recovery current” macroscopically. Although the FWD is biased reversely, a transient current flows during the reverse recovery. As the reduction rate of the reverse recovery current in shifting from the forward flow to the reverse flow is higher, the peak value of the reverse recovery current is larger. (The phenomenon is called the “hard recovery”.)
The reverse recovery current, the peak value thereof is large, is caused when the minority carriers (holes) are extracted (or swept out) from the anode electrode that is on the negative side, when the FWD is biased reversely. The reverse recovery current, the peak value thereof is large and that is caused as described above, localizes to curved portion 13 in the edge area of the p-type anode diffusion region, thereto the reverse bias electric field is liable to localize. It is well known that the reverse recovery current that behaves as described above causes a high current density in curved portion 13, further causing the breakdown of the FWD (especially when the reduction rate of the reverse recovery current in shifting from the forward flow to the reverse flow is large). The phenomena described above is caused, since the edge area of the p-type anode diffusion region has a certain curvature and, therefore, the equipotential curve density therein is liable to be dense. The phenomenon described above is caused, also since the minority carriers, distributing below edge termination region 6 formed in the surface portion of a peripheral region surrounding the p-type anode diffusion region, through which the main current of the general FWD flows, are extracted (swept out) intensively through the curved portion.
Japanese Patent Publication No. 3444081 describes a structure for relaxing the minority carrier extraction from the curved portion in the edge area of the anode diffusion region and further for lowering the reverse recovery current peak. The structure described in the Japanese Patent Publication No. 3444081 includes an anode electrode formed above the edge area of the anode diffusion region with an insulator film interposed between the edge area of the p-type anode diffusion region and the anode electrode. (In other words, the effective anode electrode area in contact with the p-type anode diffusion region is withdrawn from the edge area of the anode diffusion region.) And, the length of the insulator film sandwiched between the anode electrode and the edge area of the p-type anode diffusion region (that is the withdrawal length of the effective anode electrode area) is set to be longer than the minority carrier diffusion length.
Japanese Unexamined Patent Application Publication No. 2005-340528, discloses a structure that shortens the carrier lifetime locally so as not to make the currents localize to edge area 8 of p-type anode diffusion region 2.
The magnitude of the reverse recovery current is larger as the amount of the accumulated excess minority carriers on the collector side of n−-type layer 1 is larger. As the reverse recovery current is higher, a hard recovery waveform is more liable to be caused. As the hard recovery waveform of the reverse recovery current is caused, a higher initial reverse voltage is caused. As a too-high initial reverse voltage is caused, the initial reverse voltage exceeds the rated reverse voltage of the FWD to the higher side, breaking down the FWD.
Japanese Unexamined Patent Application Publication No. Hei. 8 (1996) 306937, and Japanese Unexamined Patent Application Publication No. Hei. 8 (1996) 306937, describe a cross-sectional view showing the edge area structure of an anode electrode. The cross-sectional view shows anode electrode 3 laminated above edge area 8 of p-type anode diffusion region 2 with insulator 9 interposed between anode electrode 3 and edge area 8.
However, in the diode (FWD) described in the Japanese Patent Publication No. 3444081, a potential difference is caused in the reverse recovery thereof between edge area 8 of p-type anode diffusion region 2 and the inner area of p-type anode diffusion region 2 in contact with anode electrode 3. If through-holes are caused by some kinds of defects in insulator 9 or if insulator 9 is thin locally, the surface of edge area 8 in p-type anode diffusion region 2 and the portion of anode electrode 3 above edge area 8 of p-type anode diffusion region 2 with insulator 9 interposed between anode electrode 3 and edge area 8 will be short-circuited with each other sometimes by insulation breakdown. Since the minority carriers distributing in the outer margin (edge termination region 6) of the diode localize to the short-circuited portion, the effects of relaxing the reverse recovery current peak described in Japanese Patent Publication No. 3444081 are not obtained so well as expected and the device will be broken down with a high possibility. The previously mentioned kinds of defects that cause holes in insulator film 9 occur due to foreign materials and damage caused during the wafer process. Usually, it is extremely difficult to form insulator film 9 that does not include any of the defects described above.
In view of the foregoing, it is desirable to obviate the problems described above. It would be also desirable to provide a semiconductor device, that is a pn-junction diode having a structure, in which an anode electrode is formed above the edge area of a p-type anode diffusion region and that stably exhibits a high reverse recovery withstand independently of the existence and nonexistence of the defects caused during the wafer process for manufacturing the semiconductor device.